Wildfire smoke is affecting people's sperm and embryos, studies show

Studies Link Wildfire Smoke Exposure to Changes in Sperm Quality and Embryo Development

Research from multiple institutions has examined associations between periods of elevated wildfire smoke and measures of male reproductive health as well as outcomes in in vitro fertilization cycles. The work remains preliminary, with authors noting small sample sizes and the need for additional data before drawing firm conclusions about fertility outcomes.

Jasper Kehoe, who worked as a wildland firefighter in Colorado for four seasons while also conducting research at Colorado State University, recruited participants for one such study. The project collected semen samples from 144 firefighters before, during, and after fire seasons. Kehoe stated that interest among colleagues increased once the topic of fertility was raised directly. The study results are scheduled for publication in late 2025 or 2026.

Separate analyses have focused on urban populations exposed to smoke. Researchers at Oregon Health & Science University reviewed semen samples from men receiving fertility care in Portland around the 2020 wildfire season, when the local air quality index reached levels near 500 for roughly ten days. They reported declines in sperm quantity and motility in the months after the smoke event.

A second study, published in 2025 and led by Tristan Nicholson at the University of Washington, analyzed samples from 84 men collected around smoke episodes in Seattle in 2018, 2020, and 2022. Most participants showed reductions in sperm quality and count following the exposure periods. Nicholson described the observed changes as relatively small and stated that it remains unknown whether the shifts affect pregnancy rates or overall fertility. The research team has called for expanded investigation into recovery timelines after smoke exposure.

Nicholson also noted that male factors in infertility have received less research attention than female factors in prior decades. The studies do not claim that smoke exposure is the sole or primary cause of any measured changes.

A related OHSU project reviewed IVF cycles that overlapped with the same 2020 Portland smoke period. Embryos that developed in the laboratory during the days of poorest air quality were less likely to reach the stage suitable for transfer. Cycles in which embryo development occurred after the smoke had cleared showed a median of two usable embryos, compared with fewer in the exposed group. The study authors reported that exposure before egg retrieval did not produce statistically significant differences in the same measures. Lead author Molly Kornfield stated that the laboratory used air filtration systems yet still detected smoke odor during the episode. She described the findings as consistent with known effects of longer-term air pollution but emphasized the small number of cases examined.

Current federal air quality guidance designates certain groups, including children, older adults, pregnant individuals, and people with cardiovascular or respiratory conditions, as sensitive populations with stricter recommended exposure limits. Nicholson said it is not yet determined whether people attempting conception should be included in that category on the basis of existing evidence. Both Nicholson and Kornfield advised minimizing exposure during periods of poor air quality while awaiting further data.

Public health agencies recommend monitoring conditions through sources such as AirNow.gov. Common exposure-reduction steps include limiting time outdoors, closing windows and doors, and using N95 respirators when outside air quality is poor. Individuals in frequently affected regions sometimes install indoor air filtration units or construct simple filters with box fans and high-efficiency particulate air filters. Some municipalities operate lending programs for such equipment during fire season.

Kehoe reported adopting personal practices such as frequent hand and face washing during assignments and running air purifiers continuously at his residence after completing his fieldwork. The studies cited do not establish permanent effects or quantify population-level changes in birth rates.

The research described here addresses associations observed during specific smoke events rather than long-term trends or causation. Authors across the projects have stated that larger, prospective studies are required to assess whether the documented changes in sperm parameters or embryo viability persist or influence clinical pregnancy outcomes.